1. Field of the Invention
The present invention relates to a process for producing a urea resin and more particularly to a process for producing (1) a urea (hereinafter designated as U-formaldehyde (hereinafter designated as F) resin having a good dispersibility and particle size of 1 .mu.m or less mixed with substantially water-insoluble inorganic particles having a wide particle size distribution or (2) a U-F resin with a low content of water-soluble ingredients which is mixed with particles of substantially water-insoluble inorganic compounds having a wide particle size distribution.
The inorganic compound-containing urea resin obtained according to the invention is useful as; a filler having high dispersibility for natural rubbers, synthetic rubbers, polyethylene, and other thermoplastic resins; filler for paper; carrier for herbicides, disinfectants, insecticides, and the like; slow-release nitrogenous fertilizer, paint pigment, heat insulator etc.
As fillers for paper, a variety of inorganic compounds have hitherto been used and there has been a recent tendency to use occasionally organic compounds such as urea resins, but this replacement has not fully been achieved. The inorganic compound-containing urea resin of the present invention is expected to be used as a good filler for paper in that the resin combines advantages of organic and inorganic compounds, contains less watersoluble matter, and can be produced at low costs.
2. Description of the Prior Art
A number of processes have been proposed for producing U-F resin particles. Examples of the processes proposed are as follows:
(1) A process comprising treating in aqueous U-F precondensate solution with an acid substance to form a crosslinked gel, followed by size reduction, drying, and crushing the gel (Japanese patent publication Nos. 28087/71, 36879/72, and 2030/74 and Japanese patent application Laid-Open No. 2953/71).
(2) A process comprising mixing an amino resin precondensate solution with a condensation catalyst in a tube mixer for a shorter residence than the time for gelling the solution, and gelling the mixture in an extruder or a screw mixer, followed by post-curing of the solid gel for a period of at least 30 minutes (Japanese patent publication No. 2350/74).
(3) A process comprising preparing a urea resin having a specific water-solubility and a specific viscosity, and adding an inorganic acid to the resin within a prescribed range of the resin-to-water ratio (Japanese patent application Laid-Open No. 82195/75).
(4) A process comprising mixing a gelling agent with an aqueous solution of a U-F precondensate having a specific F/U molar ratio, the solution having a specific water/solid ratio, and solidifying the mixture continuously over a revolving endless belt, followed by coarse grinding, slurrying, neutralizing, and grinding to form granules (Japanese patent application Laid-Open No. 135893/79).
(5) A process comprising reacting a U-F precondensate in an aqueous acid catalyst solution, followed by neutralizing the resulting mixture and adding an inorganic filler, to yield a U-F resin filler (Japanese patent application Laid-Open No. 133146/82).
For producing particles of inorganic compounds such as silica, alumina, and silica-alumina, there have also been proposed a number of processes, which can be classified into wet processes and dry processes.
A typical example of the wet processes comprises decomposing an alkali silicate, alkali aluminate, or aluminum sulfate with an inorganic acid, followed by separation, drying, and calcining the decomposition product. A typical example of the dry processes comprises vaporizing purified silicon tetrachloride or anhydrous aluminum chloride, and hydrolyzing the vapor in an oxyhydrogen flame.
According to any of the prior art processes including those described above, a U-F resin and an inorganic compound such as silica are produced separately in the form of particles. Hence, the preparation of mixtures of these products is inferior in production efficiency, admitting of improvement in this respect.
When an aqueous solution of a U-F precondensate is gelled simply by adding a gelling agent, the resulting U-F resin has generally particle sizes ranging from 1 .mu.m to scores of .mu.ms though some of the particles have sizes less than 1 .mu.m. Although these particles can be ground to sizes of 0.1 to 1 .mu.m, this requires many hours, being inferior in effectiveness. This method has an additional drawback in that, in order to provide a wide particle size distribution, it is necessary to adjust the particle sizes by mixing particle groups different in particle size distribution.
There are similar drawbacks in the preparation of particles of inorganic compounds (hereinafter sometimes referred to as inorganic particles) such as those of silica. For instance, while the size of silica particles produced by the wet process depends on various factors including the concentration and viscosity of the starting material, gelling conditions (such as the kind and amount of gelling agent, gelling temperature, agitation of reaction mixture, and ageing period of time), drying temperature, calcining temperature, etc., the sizes of silica particles produced under definite conditions are distributed in such a narrow range as from 0.01 to 0.05 .mu.m, from 1 to 10 .mu.m, or from 10 to 50 .mu.m; hence, in order to provide silica particles having fine sizes and a wide particle size distribution, for example, from 0.01 to 1 .mu.m, it is necessary to adjust the particle sizes by either mixing products different in particle size or grinding a product again.
Thus, the production of U-F resin particles and inorganic particles has required many operation steps and has been bothersome and time-consuming. Therefore, it is desired to ameliorate the production process.
On the other hand, U-F resin particles produced according to prior art processes contain much water-soluble matter, since the gelling agent used is water-soluble and even when the gelling agent remaining in the produced resin particles is neutralized with a common alkali, the salt formed thereby is generally water-soluble.
When such U-F resins are used as fillers for paper in paper making processes, water-soluble ingredients thereof are dissolved in working water and this may cause troubles. Also when such U-F resins are used as fertilizers, water-soluble salts therefrom are liable to hinder the germination of seeds and the growth of plants. Therefore it is desirable that U-F resins for the present purposes contain minimized amounts of water-soluble matter.
In order to remove water-soluble matter from the produced resin, it is necessary to add the step of washing the resin with water. Liquid formed by the separation or water used for the washing in this step may be recycled for reuse but this raises the problem of the accumulation of water-soluble matter which will deteriorate the product quality.
According to a process, as described in Japanese patent application Laid-Open No. 133146/82, cited in 5) above, which comprises reacting a U-F precondensate in an aqueous acid catalyst solution, followed by treating the resulting mixture with an alkali such as aqueous sodium hydroxide or aqueous ammonia, a water-soluble salt such as sodium sulfate or ammonium sulfate is formed by acid-alkali neutralization reaction even when such a water-insoluble inorganic filler is added as talc, clay, white carbon, titanium dioxide, calcium carbonate, calcium silicate, or aluminum hydroxide. Hence, the step of washing the separated resin with water must be added in order to remove the water-soluble salt and facilities are necessary for the disposal of washing waste water and for the prevention of environmental pollution.
The problem will not be solved even by taking the method of reacting a U-F precondensate in an aqueous acid catalyst solution, separating the resulting resin and an added inorganic filler from the mother liquor, and then treating the resin and the filler with a common alkali. It is because the thus obtained solid consists of fine particles and hence has a water-holding capacity as high as about 40 to 80% when centrifuged in the ordinary manner, so that the acid contained in the mother liquor held by the particles forms a large amount of water-soluble salt when neutralized with the alkali.